They hope studying the monkeys will allow for greater knowledge of Huntington's and ideas for new drugs.

Disease's essence

"Rodent species can capture some of the characteristics of the disease, but they have not been satisfactory in being able to really capture the essence of the disease," Dr Stuart Zola, head of the Yerkes centre, says.

"Now we have a genetically modified nonhuman primate that really has captured the clinical signs that we see in patients with Huntington's disease."

Drugs can help manage symptoms but do not stop the physical and mental decline. People typically die within 10 to 15 years after symptoms arise.

The researchers say they chose Huntington's as the disease for creating the genetically modified monkeys with an eye toward simplicity, because it is linked to mutations in a single gene rather than multiple genes.

Zola says the achievement could pave the way for creating genetically modified primates with other neurodegenerative ailments such as Parkinson's disease and Alzheimer's disease.

"This research allows scientists to advance beyond mouse models, which do not replicate all of the changes in the brain and behaviour that humans with Huntington's disease experience," says Dr John Harding, a primate resources official at the National Institutes of Health, which funded the study.

Viral vector technology

Using so-called viral vector technology, the researchers transferred the Huntington's gene into a monkey egg cell.

After using in vitro fertilisation, the egg grew into a four-cell embryo and was then placed in the womb of a female monkey acting as a surrogate mother.

Of the five baby monkeys born using this process, two died within about a day, another one died in about a month and two are still living at age 10 months, according to Dr Anthony Chan of the Yerkes centre and Emory University School of Medicine.

One of the two surviving monkeys has developed symptoms including involuntary movements of the hands and face, Chan says.

The other has no symptoms of the disease yet but may develop them later, he says.